Thermal interface material application for integrated circuit cooling

ABSTRACT

Techniques provide improved thermal interface material application in an assembly associated with an integrated circuit package. For example, an apparatus comprises an integrated circuit module, a printed circuit board, and a heat transfer device. The integrated circuit module is mounted on a first surface of the printed circuit board. The printed circuit board has at least one thermal interface material application via formed therein in alignment with the integrated circuit module. The heat transfer device is mounted on a second surface of the printed circuit board and is thermally coupled to the integrated circuit module. The second surface of the printed circuit board is opposite to the first surface of the printed circuit board.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Divisional of U.S. patent application Ser. No.12/882,362, filed on Sep. 15, 2010, the disclosure of which is fullyincorporated herein by reference.

FIELD

The field relates generally to integrated circuit package cooling and,more particularly, to techniques for providing improved thermalinterface material application in an assembly including an integratedcircuit package.

BACKGROUND

In today's portable electronic devices, there are typically one or moreradio frequency (RF) modules that serve to provide wireless dataexchange between the device and its operating environment. Such an RFmodule requires one or more antennas to transmit/receive data signals.The RF module and antennas are mounted in some manner on a printedcircuit board (PCB).

Until recently, the one or more antennas have been designedindependently as a printed shape on the (PCB) or as an individualcomponent to be assembled near an RF integrated circuit (RFIC) die.However, the assembly of the individual antenna or antennas on the boardis problematic in the context of mass production.

Recently, it has become popular to integrate the one or more antennasinto the RF module. It is understandable that the integration of the oneor more antennas into the module brings a huge advantage in terms ofcost and performance. However, cooling becomes very challenging sincecooling devices used to transfer heat away from the RF module, such asheat sinks and heat spreaders, should be mounted so as not toelectromagnetically interfere with the signal transmission/reception ofthe one or more antennas.

To address this interference problem, cavity-down type integratedcircuit packages have been proposed. In such a package, theantenna-embedded package has a cavity formed on its bottom surface inwhich the RFIC die is mounted. This cavity-down type integrated circuitpackage is mounted on a top surface of the PCB with a heat sink or heatspreader mounted below the PCB. This way, the heat sink or heat spreaderdoes not electromagnetically interfere with the signaltransmission/reception of the one or more antennas. However, there mustbe some type of heat transfer mechanism/medium employed between thecomponents to effectively transfer heat away from the RFIC die.

SUMMARY

Techniques provide improved thermal interface material application in anassembly including an integrated circuit package.

For example, in a first aspect of the invention, an apparatus comprisesan integrated circuit module, a printed circuit board, and a heattransfer device. The integrated circuit module is mounted on a firstsurface of the printed circuit board. The printed circuit board has atleast one thermal interface material application via formed therein inalignment with the integrated circuit module. The heat transfer deviceis mounted on a second surface of the printed circuit board and isthermally coupled to the integrated circuit module. The second surfaceof the printed circuit board is opposite to the first surface of theprinted circuit board.

In one embodiment, the integrated circuit module comprises at least oneradio frequency integrated circuit (RFIC) die and at least one embeddedantenna package. The at least one RFIC die and the at least one embeddedantenna package may be electrically coupled via a flip-chip typeconnection or via a wire- bond type connection. Further, the at leastone embedded antenna package may comprise a substrate and at least oneantenna embedded in the substrate. The substrate may comprise an organicmaterial or a ceramic material. The at least one embedded antennapackage may comprise a surface mounting feature for mounting to thefirst surface of the printed circuit board, wherein the surface mountingfeature of the substrate is one of a ball grid array, a land grid array,and a quad flat package. Still further, the integrated circuit modulemay be a cavity-down type integrated circuit package.

In a second aspect of the invention, an antenna assembly comprises anantenna package, a printed circuit board, and a heat transfer device.The antenna package is mounted on a first surface of the printed circuitboard. The printed circuit board has at least one thermal interfacematerial application via formed therein in alignment with the antennapackage. The heat transfer device is mounted on a second surface of theprinted circuit board and is thermally coupled to the antenna package.The second surface of the printed circuit board is opposite to the firstsurface of the printed circuit board. In one embodiment, the antennapackage is a millimeter wave antenna package.

In a third aspect of the invention, a method comprises the followingsteps. At least one thermal interface material application via is formedin a printed circuit board. An integrated circuit module is mounted onthe printed circuit board. The integrated circuit module is mounted on afirst surface of the printed circuit board in alignment with the atleast one thermal interface material application via formed therein. Aheat transfer device is mounted on a second surface of the printedcircuit board in alignment with the at least one thermal interfacematerial application via formed therein. The second surface of theprinted circuit board is opposite to the first surface of the printedcircuit board.

In one embodiment, the method further comprises applying a thermalinterface material through the at least one thermal interface materialapplication via formed in the printed circuit board prior to mountingthe heat transfer device on a second surface of the printed circuitboard. The thermal interface material may be applied through the atleast one thermal interface material application by injecting thethermal interface material through the at least one thermal interfacematerial application via. Further, the thermal interface material may beapplied through the at least one thermal interface material applicationvia formed in the printed circuit board after one or more components aresurface-mounted on the printed circuit board. The one or more componentsmay be surface-mounted on the printed circuit board via a reflowsoldering process.

In a fourth aspect of the invention, a method comprises the followingsteps. At least one non-plated through via is formed in a printedcircuit board. An integrated circuit module is mounted on the printedcircuit board. The integrated circuit module is mounted on a firstsurface of the printed circuit board in alignment with the at least onenon-plated through via formed therein. One or more components aremounted on the printed circuit board via a reflow soldering process. Athermal interface material is injected through the at least onenon-plated through via. A heat transfer device is mounted on a secondsurface of the printed circuit board in alignment with the at least onenon-plated through via formed therein. The second surface of the printedcircuit board is opposite to the first surface of the printed circuitboard.

Advantageously, the above-described techniques provide for improvedapplication of thermal interface material in an integrated circuitpackage assembly such that the thermal interface material is notdegraded or otherwise compromised by other assembly steps. For example,illustrative embodiments of the invention provide efficient coolingpaths within an RF module between a cooling device (heat transferdevice) and an RFIC. With the inventive fabrication and structuralarrangements, the efficiency of heat removal is greatly improved.

These and other objects, features, and advantages of the presentinvention will become apparent from the following detailed descriptionof illustrative embodiments thereof, which is to be read in connectionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a placement on a printed circuit boardof a heat transfer device with respect to an integrated circuit package,in accordance with one embodiment of the invention.

FIG. 2 illustrates a cavity-down type integrated circuit package, inaccordance with one embodiment of the invention.

FIG. 3 illustrates a printed circuit board assembly with a cavity-downtype integrated circuit package and heat transfer device, in accordancewith one embodiment of the invention.

FIGS. 4A through 4E illustrate a method of fabricating the printedcircuit board assembly of FIG. 3.

DETAILED DESCRIPTION

Principles of the present invention will be described herein in thecontext of illustrative integrated circuit packages such as acavity-down type integrated circuit package and illustrative integratedcircuit dies such as a radio frequency integrated circuit (RFIC) die.However, it is to be appreciated that the principles of the presentinvention are not limited to any particular package type or IC die.Rather, the principles of the invention are directed broadly totechniques for improved thermal interface material application in thefabrication process of a printed circuit board assembly that includes anintegrated circuit package and a heat transfer device. While principlesof the invention are not limited to any particular package or die types,they are well-suited for use in millimeter (mm) wave antenna assemblies.

As will be illustratively described herein, in the context of variousillustrative embodiments, principles of the invention provide techniquesthat provide efficient dissipation of heat generated by a semiconductordie such as an RFIC die.

Recall, as mentioned above, that when one or more antennas areintegrated into an RFIC package, such as a cavity-down type integratedcircuit package, cooling becomes very challenging since heat transferdevices used to transfer heat away from the RF module, such as heatsinks and heat spreaders, should be mounted so as not toelectromagnetically interfere with the signal transmission/reception ofthe one or more antennas.

FIG. 1 illustrates an example of a placement of components on a printedcircuit board that accomplishes a goal of eliminating electromagneticinterference caused by the heat transfer device.

As shown, the printed circuit board assembly 100 in FIG. 1 comprises anintegrated circuit package 102, which could be an RFIC module that hasan embedded antenna array (e.g., a mmWave package), mounted on a first(e.g., top) surface of a printed circuit board 104. The assembly 100also comprises a heat transfer device (heat sink or heat spreader) 106mounted on a second (e.g., bottom) surface of the printed circuit board104.

With the heat transfer device 106 mounted on the side of the printedcircuit board 104 opposite to the side of the printed circuit board towhich the integrated circuit package 102 is mounted, the embeddedantenna array in the integrated circuit package 102 is able totransmit/receive signals without experiencing electromagneticinterference due to the heat transfer device 106.

FIG. 2 illustrates an example of a cavity-down type integrated circuitpackage, in accordance with one embodiment of the invention. Thecavity-down type integrated circuit package 200 shown in FIG. 2 is oneexample of an integrated circuit package 102 (FIG. 1).

As shown, the cavity-down type integrated circuit package 200 comprisesan antenna array package 202 and an RFIC die 204. The antenna arraypackage 202 comprises a substrate 203. The substrate 203 can comprise anorganic material such as, by way of example only, liquid-crystalpolymer, polytetrafluoroethylene, or an FR4 based laminate.Alternatively, the substrate 203 can comprise a ceramic material.

An antenna (or an antenna array with more than one antenna) 206 isembedded at the top part of the substrate 203. The RFIC die 204 and aball grid array (BGA) 212 (or any other surface mount packages such asland grid array (LGA) and quad flat package (QFP)) are attached to thebottom side of the substrate 203.

In this embodiment, the RFIC die 204 is flip-chip mounted so that thebackside 205 of the die is exposed and available for heat removal. Aflip-chip type connection is a method for interconnecting semiconductordevices, such as integrated circuit dies, to external circuitry withsolder bumps that are deposited onto the chip pads. These solder bumps(210 in FIG. 2) electrically connect with copper pads 208 of theexternal circuitry, in this case, the antenna array package 202. Thecopper pads are electrically connected with the one or more antennas206. The solder bumps 210 are typically deposited on the chip pads onthe top side 207 of the die 204 during the final die processing step.Thus, in order to mount the RFIC die 204 to the antenna array package202, the die is flipped over so that it faces down, and aligned so thatits pads align with matching pads on the antenna array package 202, andthen the solder is flowed to complete the interconnect.

This is in contrast to wire bonding, in which the chip is mountedupright and wires are used to interconnect the chip pads to externalcircuitry. Such a wire-bond type connection may alternatively be used toconnect the RFIC die 204 and the antenna array package 202.

FIG. 3 illustrates a printed circuit board assembly with a cavity-downtype integrated circuit package and heat transfer device, in accordancewith one embodiment of the invention. The printed circuit board assembly300 in FIG. 3 comprises a cavity-down type integrated circuit package302 mounted on a top surface of a printed circuit board 304, and a heattransfer device (cooling device) 306 mounted on the bottom surface ofthe printed circuit board 304. Note that the cavity-down type integratedcircuit package 302 corresponds to the cavity-down type integratedcircuit package 200 in FIG. 2.

As shown in FIG. 3, the printed circuit board 304 comprises at least onethermal interface material application via 307 formed in alignment withthe integrated circuit package 302 and the heat transfer device 306.While only one via 307 is shown in FIG. 3, it is understood that morethan one such via 307 may be formed in the printed circuit board 304.Via 307 is preferably non-plated. The printed circuit board 304 also hasone or more plated thermal vias 308 that connect copper pads 309 withthe heat transfer device 306. These plated thermal vias 308 are alsopositioned in alignment with the integrated circuit package 302 and theheat transfer device 306.

Also as shown, a thermal interface material (TIM) 305 is in contact withthe RFIC die of the integrated circuit package 302 and the copper pads309. As will be explained below in the fabrication process of FIGS. 4Athrough 4E, the TIM 305 is applied by injecting the TIM through thebottom of the thermal interface material application via 307 before theheat transfer device 306 is mounted to the bottom of the printed circuitboard 304. Examples of TIM may include, but are not limited to, siliconeoil filled with aluminum oxide, zinc oxide, or boron nitride, micronizedor pulverized silver, and phase-change materials. TIM is used as a heattransfer medium that allows heat energy to move from the RFIC die to thecopper pads 309 and plated thermal vias 308 of the printed circuit board304 to the heat transfer device 306.

FIGS. 4A through 4E illustrate a method of fabricating the printedcircuit board assembly of FIG. 3.

As shown in FIG. 4A, a ball grid array 404 is attached to the bottom ofthe substrate that is part of the integrated circuit package 402 (i.e.,package 302 in FIG. 3). This is referred to as the BGA balling process.

Next, as shown in FIG. 4B, the integrated circuit package 402 is mountedon the top surface of the printed circuit board 406 (i.e., printedcircuit board 306) with the thermal interface material applicationvia(s) 407 (i.e., thermal interface material application via(s) 307)formed therein. The integrated circuit package 402 is electricallyconnected to the printed circuit board 406 by a reflow solderingprocess, in this case, a BGA reflow. As is known, reflow soldering is aprocess in which a solder paste, such as an adhesive-like mixture ofpowdered solder and flux, is used to temporarily attach one or moreelectrical components to their contact pads, after which the entireassembly is subjected to controlled heat (e.g., reflow oven or someother heat source). The controlled heat melts the solder therebypermanently connecting the joint(s).

Then, as shown in FIG. 4C, one or more components 410 aresurface-mounted to the printed circuit board 406. These components maybe other IC chips that are part of the printed circuit board. They aretypically surface-mounted using a reflow soldering process (similar tothe one described above for BGA reflow), in this case, an SMT (surfacemount technology) reflow.

In FIG. 4D, thermal interface material (TIM) 412 (i.e., TIM 305) isinjected through the bottom of the thermal interface materialapplication via 407 using a syringe (not shown). The TIM 412 contactsthe RFIC die of the integrated circuit package 402 and provides athermal conduit between the RFIC die and the heat transfer device 414(i.e., heat transfer device 306), which is then mounted on the bottomsurface of the printed circuit board 406, as shown in FIG. 4E. The TIM412 establishes a low-thermal-resistance interface between the RFIC dieand the heat transfer device.

In existing fabrication processes, it is known that the TIM is appliedto the RFIC die and the corresponding area of the top surface of theprinted circuit board prior to the BGA reflow and the SMT reflow. Insuch cases, the TIM is exposed to solder reflow conditions severaltimes. We have realized that, when using such an existing fabricationprocess, the TIM fails to maintain suitable heat transfer propertiesafter exposure to the solder reflow. That is, the TIM becomes degradedand non-stable. As such, with existing fabrication processes, anextensive production qualification test (PQT) is required to confirm TIMstability before going into production.

However, in accordance with principles of the invention as illustratedin FIGS. 4A through 4E, the TIM is applied after the BGA and SMTreflows, which is made possible by the formation of non-platedapplication via(s) 407 in the printed circuit board 406, thus allowinginjection of the TIM after the reflow steps but before attachment of theheat transfer device. Since the TIM can be injected near the very end ofthe assembly process and need not to be exposed to solder reflow, thetime-consuming PQT processes are eliminated.

In one embodiment, the size of BGA balls is chosen to ensure that thecombined height of the flip-chip mounted RFIC die is less than the BGAstand-off. In another embodiment, the package substrate has an opencavity to accommodate the RFIC die. In such a case, the die need not bethinned. In yet another embodiment, the printed circuit board can have arecess larger than the size of the die. During the BGA reflow, the diewould slip into the recess. Still further, the package substrate and theboard substrate can be made of any materials including, but not limitedto, FR4, polytetrafluoroethylene, liquid-crystal polymer based laminatesor build-up organics, as well as ceramic substrates. Also, principles ofthe invention can be applied to any cavity-down type packages for anysemiconductor die.

It will be appreciated and should be understood that the exemplaryembodiments of the invention described above can be implemented in anumber of different fashions. Given the teachings of the inventionprovided herein, one of ordinary skill in the related art will be ableto contemplate other implementations of the invention. Indeed, althoughillustrative embodiments of the present invention have been describedherein with reference to the accompanying drawings, it is to beunderstood that the invention is not limited to those preciseembodiments, and that various other changes and modifications may bemade by one skilled in the art without departing from the scope orspirit of the invention.

What is claimed is:
 1. A method, comprising: forming at least onethermal interface material application via in a printed circuit board;mounting an integrated circuit module on the printed circuit board, theintegrated circuit module being mounted on a first surface of theprinted circuit board in alignment with the at least one thermalinterface material application via formed therein; mounting a heattransfer device on a second surface of the printed circuit board inalignment with the at least one thermal interface material applicationvia formed therein, the second surface of the printed circuit boardbeing opposite to the first surface of the printed circuit board.
 2. Themethod of claim 1, further comprising applying a thermal interfacematerial through the at least one thermal interface material applicationvia formed in the printed circuit board prior to mounting the heattransfer device on a second surface of the printed circuit board.
 3. Themethod of claim 2, wherein the thermal interface material is appliedthrough the at least one thermal interface material application byinjecting the thermal interface material through the at least onethermal interface material application via.
 4. The method of claim 2,wherein the thermal interface material is applied through the at leastone thermal interface material application via formed in the printedcircuit board after one or more components are surface-mounted on theprinted circuit board.
 5. The method of claim 4, wherein the one or morecomponents are surface-mounted on the printed circuit board via a reflowsoldering process.
 6. A method, comprising: forming at least onenon-plated through via in a printed circuit board; mounting anintegrated circuit module on the printed circuit board, the integratedcircuit module being mounted on a first surface of the printed circuitboard in alignment with the at least one non-plated through via formedtherein; mounting one or more components on the printed circuit boardvia a reflow soldering process; injecting a thermal interface materialthrough the at least one non-plated through via; and mounting a heattransfer device on a second surface of the printed circuit board inalignment with the at least one non-plated through via formed therein,the second surface of the printed circuit board being opposite to thefirst surface of the printed circuit board.